For want of one "line replaceable unit", a $73m drone was lost

We don't cover military drones here, but I'll make an exception for this fascinating accident report. We've all lost drones due to technical malfunctions, so we can sympathize with this. At least ours didn't cost $73 million.

On August 11, 2011 Air Force ground controllers lost contact with one of the military’s most advanced — and expensive — drones, an EQ-4B Global Hawk as the jet flew high over Eastern Afghanistan.

Nine hours into an otherwise smooth communications relay mission using the Battlefield Airborne Communications Node (BACN), the plane was cruising at 51,000-feet above sea level 105 nautical miles northwest of Kandahar, Afghanistan — close to the border with Pakistan, in fact, the few news reports that emerged of the crash claim the jet actually went down inside Pakistan — when a pilot from the 12th Reconnaissance Squadron out of Beale Air Force Base in California “lost all links with the payload,” according to a copy of the Air Force’s report on the incident that the service sent to DT. The pilot did everything he could to reestablish communications with the mammoth drone, but 25 seconds after losing communications, the plane began a high-speed fall to Earth. If plummeted so fast that “both wings and at least one of the lower aft fuselage fairings” were ripped off the jet as it fell. Three minutes later, the Global Hawk crashed into “remote, desert terrain approximately 4 nm from its last reported position and was destroyed,” reads the Air Force’s report. “the estimated loss is valued at 72.8 million.”

So, what caused this?

A single part — or Line Replaceable Unit, as the report calls it — came undone, interrupting the flow of electricity to the plane’s aileron and spoiler actuators — the tiny motors that control the movements of an aircraft’s flight control surfaces you know, the moving parts of the wings that control whether the plane climbs, dives, banks, rolls, etc. As expected, this disconnection rendered “the aircraft uncontrollable.”

(Critical parts that lose it like this one are called single points of failure, meaning that if these sometimes tiny and seemingly insignificant parts fail, the entire weapon system fails. Naturally, military equipment makers try o identify these and do all they can to ensure they won’t fail.)

Why did this single part become disconnected? “The board president also found, by a preponderance of the evidence, that the LRU [the critical part] installation methods were a contributing factor in the mishap,” reads the a summary of the report. Apparently, the screws holding the part in place weren’t tight enough and probably shook loose due to typical flight vibrations.

One other thing to note, the jet’s “avionics were not recovered from the crash site.” Let’s hope they were destroyed in the crash and the subsequent bombing of the wreckage by Air Force bombers and not scooped up by someone who could sell them to the Russians or Chinese.

Actually for 76mil they could get a cascade of 33 autopilots, feeding 5 separately voting arbitration systems, cascaded themselves into 3 separate voting systems, out of which the final output cable would be left loosely unconnected by a screw that normally worked fine for more complicated machines, but not after 123 landings in one specific UAV.

That way they would get heavy, complicated, costly unreliable, non-working system, but all asses would be covered.